Electrodeless low-pressure discharge lamp operating device and self-ballasted electrodeless fluorescent lamp

ABSTRACT

An electrodeless discharge lamp operating device including a light-transmitting discharge bulb  120 , an induction coil including a core  103  and a coil  104 , and a ballast circuit  140  for supplying a high-frequency power to the induction coil. The operating frequency of the ballast circuit  140  is in the range of 80 kHz to 500 kHz, and where the operating frequency of the ballast circuit  140  is f (kHz) and the power input to the discharge bulb  120  is P (W), the rare gas pressure p (Pa) in the discharge bulb  120  satisfies the relationship of the following expression:  
             p   ≥       A     P   -     B     f   2       -   C                 [     Expression   ⁢           ⁢   1     ]             
 
(where A, B and C are constants having the following values: A=4.0×10 4 , B=3.5×10 4  and C=6.2), and the power input P to the discharge bulb  120  is  7  W at minimum and  22  W at maximum.

TECHNICAL FIELD

The present invention relates to an electrodeless low-pressure dischargelamp, and more particularly to a self-ballasted electrodelessfluorescent lamp.

BACKGROUND ART

Due to the absence of electrodes, electrodeless fluorescent lamps havelonger lifetimes than fluorescent lamps with electrodes, and haveefficiencies as high as those of common fluorescent lamps. With suchcharacteristics, electrodeless fluorescent lamps have been drawingpublic attention from the point of view of environmental protection andeconomic efficiency, and have a potential for becoming more and morewidespread in the future. Electrodeless fluorescent lamps are demandedprimarily as an alternative light source replacing incandescent lamps,which have been widely used in general lighting. Where electrodelessfluorescent lamps are used for this purpose, they are required to be ascompact as incandescent lamps, have high lamp efficiencies and beeconomical.

Electrodeless fluorescent lamps, having higher efficiencies and longerlifetimes than fluorescent lamps with electrodes, can be suitable lightsources. For example, commercially-available electrodeless fluorescentlamps use operating frequencies in a MHz frequency range such as 13.56MHz, being an ISM band, the rated power of these lamps is about 25 W to150 W, and the lifetime thereof is 15,000 to 60,000 hours. It has beenshown that they have desirable maintainability and efficiency.

These electrodeless fluorescent lamps that are being sold in the markettoday are primarily used for lighting at locations where replacing lampsrequires a high cost, such as landscape lighting, street lighting,bridge lighting, public park lighting, lighting for factories with highceilings, etc., and most of them use separate ballast circuits.

In recent years, self-ballasted electrodeless fluorescent lamps havebeen developed in the art that can be plugged into incandescent-lampsockets and used as if they were incandescent lamps, while retaining theadvantageous characteristics of electrodeless fluorescent lamps such asthe high efficiencies and long lifetimes. Discussions have been made onwidely spreading self-ballasted electrodeless fluorescent lamp havingsuch advantageous characteristics as an alternative light sourcereplacing incandescent lamps. Specifically, self-ballasted electrodelessfluorescent lamps including a discharge bulb and a ballast circuitintegrated as one unit have been developed in the art and expected tobecome widespread, which can be plugged into incandescent-lamp socketsso that they can be used as an alternative light source replacingincandescent lamps at locations where incandescent lamps haveconventionally been used, such as hotels, restaurants and houses.

The electrodeless fluorescent lamps required as an incandescent lampreplacement, unlike those used for public outdoor lighting, are thosethat have a luminous flux equivalent to that of an incandescent lamp of60 W to 100 W and have a wattage of about 10 W to 20 W. There is ademand for these low-wattage electrodeless fluorescent lamps as anincandescent lamp replacement to not only have long lifetimes but alsobe compact, readily acceptable pricewise, and free of electromagneticinterference (EMI) with surrounding electric appliances.

A primary object of the present invention, which has been made in viewof the above, is to provide an electrodeless discharge lamp operatingdevice that exhibits desirable characteristics (particularly,maintaining a stable discharge) even in an electrodeless discharge lampoperating device in which electromagnetic interference (EMI) issuppressed.

DISCLOSURE OF THE INVENTION

An electrodeless low-pressure discharge lamp operating device of thepresent invention includes: a light-transmitting discharge bulb filledwith a rare gas including at least krypton and mercury; an inductioncoil including a core and a coil wound around the core for generating anelectromagnetic field inside the discharge bulb; and a ballast circuitfor supplying a high-frequency power to the induction coil, wherein: anoperating frequency of the ballast circuit is in a range of 80 kHz to500 kHz, and where the operating frequency of the ballast circuit is f(kHz) and a power input to the discharge bulb is P (W), a pressure p(Pa) of the rare gas in the discharge bulb satisfies a relationship of afollowing expression: $\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$(where A, B and C are constants having the following values: A=4.0×10⁴,B=3.5×10⁴ and C=6.2); and the power input P to the discharge bulb is 7 Wat minimum and 22 W at maximum.

Herein, the “low pressure” as in the “electrodeless low-pressuredischarge lamp operating device” means that the pressure in thedischarge bulb is lower than that of an HID lamp (High IntensityDischarge lamp), e.g., a high-pressure mercury lamp or a high-pressuresodium lamp. Specifically, it means that the pressure of the substancefilled in the discharge bulb during the stable operation period is 1 kPaor less.

A self-ballasted electrodeless fluorescent lamp of the present inventionincludes: a light-transmitting discharge bulb filled with a rare gasincluding at least krypton and mercury; an induction coil including acore and a coil wound around the core and being inserted into a cavityportion provided in a portion of the discharge bulb; a ballast circuitfor supplying a high-frequency power to the induction coil; and a baseelectrically connected to the ballast circuit, wherein: an operatingfrequency of the ballast circuit is in a range of 80 kHz to 500 kHz, andwhere the operating frequency of the ballast circuit is f (kHz) and apower input to the discharge bulb is P (W), a pressure p (Pa) of therare gas in the discharge bulb satisfies a relationship of a followingexpression: $\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$(where A, B and C are constants having the following values: A=4.0×10⁴B=3.5×10⁴ and C=6.2); and the power input P to the discharge bulb is 7 Wat minimum and 22 W at maximum.

In one embodiment, the core of the induction coil contains iron,manganese and zinc.

In one embodiment, the rare gas filled in the discharge bulb includesargon; and the argon is 10% or more and 50% or less of the rare gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a testing device for testingelectrodeless discharge lamp operating characteristics.

FIG. 2 is a graph illustrating the relationship between the input powerand the total luminous flux.

FIG. 3 is a three-dimensional plot of the discharge maintaining powerP_(min) with respect to the gas pressure p and the operating frequencyf.

FIG. 4(a) is a graph illustrating the relationship between the gaspressure p and the discharge maintaining power P_(min), and FIG. 4(b) isa graph illustrating the relationship between 1/p² and the dischargemaintaining power P_(min).

FIG. 5 is a contour map of the discharge maintaining power P_(min) withrespect to the gas pressure p and the operating frequency f.

FIG. 6 is a cross-sectional view schematically illustrating aconfiguration of a self-ballasted electrodeless fluorescent lampaccording to an embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration of a ballast circuitfor a self-ballasted electrodeless fluorescent lamp according to anembodiment of the present invention.

FIG. 8 shows the relationship between the krypton gas pressure and thelamp efficiency of a self-ballasted electrodeless fluorescent lampaccording to an embodiment of the present invention.

FIG. 9 shows the relationship between the argon gas mixing ratio and thetotal luminous flux in a self-ballasted electrodeless fluorescent lampaccording to an embodiment of the present invention.

FIG. 10 shows the relationship between the argon gas mixing ratio andthe luminous flux one second after the starting in a self-ballastedelectrodeless fluorescent lamp according to an embodiment of the presentinvention.

FIG. 11 is a table showing the discharge maintaining power valuesobtained from the gas pressure and the operating frequency.

FIG. 12 is a table showing the relationship between the gas pressure andthe discharge maintaining power where the operating frequency is 423kHz.

BEST MODE FOR CARRYING OUT THE INVENTION

Before describing an embodiment of the present invention, basicresearches performed by the present inventors before completing theinvention will be described, after which an electrodeless low-pressuredischarge lamp operating device and a self-ballasted electrodelessfluorescent lamp according to the embodiment of the present inventionwill be described. Note that the terms “electrodeless discharge lamp”and “electrodeless discharge lamp operating device” will hereinafterrefer to an “electrodeless low-pressure discharge lamp” and an“electrodeless low-pressure discharge lamp operating device”,respectively.

In order to develop an electrodeless fluorescent lamp as an incandescentlamp replacement primarily for use in hotels, houses, etc., the presentinventors produced and lit prototypes of low-wattage electrodelessfluorescent lamps with operating frequencies of 500 kHz or less andwattages of 20 W or less for characteristics evaluation and visualobservation thereof. As a result, it was revealed that an unexpectedphenomenon occurs that had not been observed with high-wattage (e.g.,150 W) electrodeless discharge lamps used primarily outdoors. Thephenomenon is as follows. In a low-wattage electrodeless fluorescentlamp in which the input power to the discharge bulb is about 10 W to 20W, when the buffer gas pressure is set to a value of about 40 to 50(Pa), which is a value used in a high-wattage (e.g., 150 W)electrodeless discharge lamp, the discharge is likely to be veryunstable and the lamp cannot be operated in some cases.

Then, the present inventors produced prototypes of low-wattageelectrodeless discharge lamps aiming at avoiding such a phenomenon, andobtained conditions under which the lamps can be prevented fromflickering or going out and a stable discharge can be maintained, thuscompleting the present invention.

The researches performed by the present inventors will be describedbelow in detail. Where the type of the gas to be filled in and the shapeof the discharge bulb are given, whether or not a discharge in anelectrodeless discharge lamp can be maintained is dependent primarily onthe pressure p of the fill gas and the electric field strength E in thedischarge bulb. Under a condition where a discharge is being maintained,it can be considered that the product n_(n)·ν_(e) between the numbern_(n) of neutral particles in the discharge bulb and the electroncollision frequency ν_(e) is substantially constant or, in other words,the product pE between the rare gas pressure p and the electric fieldstrength E is substantially constant. Thus, with an increased pressure pof the rare gas to be filled in, it is possible to maintain a dischargeeven with a low electric field strength E.

Moreover, the relationship between the power input P to the dischargebulb of an electrodeless discharge lamp and the electric field strengthE can be given by the following expression: $\begin{matrix}{{P_{in} \approx {\sigma\quad E^{2}}} = {\frac{e^{2}n_{e}}{m_{e}V_{e}} \cdot E^{2}}} & \left\lbrack {{Expression}\quad 2} \right\rbrack\end{matrix}$where σ is the conductivity, e the electron charge, n_(e) the electrondensity, and m_(e) the mass of an electron.

As can be seen based on this expression and that the product pE betweenthe rare gas pressure p and the electric field strength E can beconsidered substantially constant, the following expression is obtained:$\begin{matrix}{P_{\min} \propto \frac{1}{p^{2}}} & \left\lbrack {{Expression}\quad 3} \right\rbrack\end{matrix}$for the minimum power input P_(min) required for maintaining a discharge(hereinafter referred to simply as the “discharge maintaining power”)and the rare gas pressure p.

Moreover, the electric field strength E in the discharge bulb based onthe induced magnet field produced by an induction coil of anelectrodeless discharge lamp operating device is proportional to thefrequency of the induced current, i.e., the operating frequency f of theelectrodeless discharge lamp operating device. Thus, the relationshipbetween the discharge maintaining power P_(min) (W) of the electrodelessdischarge lamp and the operating frequency f thereof is given by thefollowing expression: $\begin{matrix}{P_{\min} \propto \frac{1}{f^{2}}} & \left\lbrack {{Expression}\quad 4} \right\rbrack\end{matrix}$

The present inventors derived, based on Expression 3 and Expression 4above, that the discharge maintaining power P_(min) (W) of anelectrodeless discharge lamp can be approximated as shown in Expression5 below: $\begin{matrix}{P_{\min} = {{A\quad\frac{1}{p^{2}}} + {B\quad\frac{1}{f^{2}}} + C}} & \left\lbrack {{Expression}\quad 5} \right\rbrack\end{matrix}$where p (Pa) is the rare gas pressure, and f (kHz) the operatingfrequency. Herein, A, B and C are constants.

As can be seen from Expression 5, the value of the discharge maintainingpower P_(min) increases as the rare gas pressure p is lowered. Thismeans that with lamps of lower wattages, it becomes more difficult tomaintain a discharge as the rare gas pressure is lowered. Thus, it canbe understood qualitatively that while a stable discharge can bemaintained even when the krypton gas pressure is set to 40 to 50 Pa withcommercially-available high-wattage-type (e.g., 100 W) electrodelessfluorescent lamps, a discharge may become unstable or difficult to bemaintained under such a low gas pressure with low-wattage (e.g., 13 W)electrodeless discharge lamps. It can also be seen that phenomena suchas flickering are even more likely to occur with electrodeless dischargelamps in which the operating frequency is lowered to be about a few 100kHz, as an EMI countermeasure, from the MHz range, which is used forconventional electrodeless discharge lamps.

In view of this, the present inventors produced prototypes ofelectrodeless discharge lamps as an incandescent lamp replacement, andconducted experiments to examine how the discharge maintaining powerP_(min) changes as the fill gas pressure and the operating frequency ofthe ballast circuit are varied. The details of such an experiment as anexample will now be described, together with the conditions and resultsof the experiment.

FIG. 1 is a basic configuration diagram of a testing device forexamining the operating characteristics of the electrodeless dischargelamp used in the present experiment. The testing device illustrated inFIG. 1 includes an electrodeless discharge lamp 260 and a ballastcircuit 440.

The electrodeless discharge lamp 260 includes a light-transmittingdischarge bulb 120 and an induction coil 130. The induction coil 130 isa member for supplying a high-frequency power from the ballast circuit440 to the discharge bulb 120.

As illustrated in FIG. 1, the discharge bulb 120 includes an outer tube101 and an inner tube 102, with an exahust tube 105 connected to theinner tube 102. Mercury and krypton as a rare gas (not shown) are filledin the discharge bulb 120, and a phosphor layer (not shown) is formed byphosphor coating on the inside of the discharge bulb 120. The phosphorlayer serves to convert, to a visible light radiation, an ultravioletradiation generated through the excitation of mercury filled in thedischarge bulb 120.

The induction coil 130 is provided between the inner tube 102 of thedischarge bulb 120 and the exahust tube 105. The induction coil 130,made of a magnetic material (soft magnetic material), includes agenerally tubular ferrite core 103 and a winding 104. The winding 104 isconnected to the ballast circuit 440, which is a circuit for supplying ahigh-frequency current to the induction coil 130.

Note that the outer tube 101 of the discharge bulb used in the presentexperiment has a diameter D1 of 65 mm and a height H1 of 75 mm, and theinner tube 102 has an outer diameter D2 of 20 mm and a height H2 of 63mm. Moreover, the core 103 of the induction coil 130 has a length H3 of55 mm, an outer diameter D3 of 14 mm and an inner diameter D4 of 6 mm,and the number of turns of the winding 104 is 66.

As illustrated in FIG. 1, the ballast circuit 440 includes an oscillator410, an amplifier circuit 420 and a matching circuit 430. The oscillator410 functions to set the frequency of the high-frequency power suppliedto the discharge bulb 120, the amplifier circuit 420 functions toamplify the power from the oscillator 410, and the matching circuit 430functions to match the output from the amplifier circuit 420 with theimpedance of the electrodeless discharge lamp 260.

In the present experiment, the operating frequency of the ballastcircuit 440 was set by the oscillator 410 to a frequency in the range of100 kHz to 140 kHz and the pressure of the krypton gas filled in as arare gas was varied over the range of 120 Pa to 240 Pa, so as to obtainthe minimum power required to be supplied to the discharge bulb 120 formaintaining a stable discharge, i.e., the discharge maintaining powerP_(min) (W), for each combination of the operating frequency of the gaspressure. The discharge maintaining power P_(min) as used hereinincludes not only the power consumed by a discharge plasma but also thepower loss through the induction coil 130, and is the power supplied tothe induction coil (the power is hereinafter referred to as “the powerinput to the discharge bulb”).

FIG. 11 shows an example of the results of the present experiment. FIG.11 shows the values of the discharge maintaining power P_(min) (W) wherethe operating frequency f of the ballast circuit 440 was varied over therange of about 90 kHz to 145 kHz while the pressure p of the krypton gasfilled in the discharge bulb 120 was set to 120, 140, 160 or 240 Pa.

P_(min) (W) in FIG. 11 can be obtained as shown in FIG. 2. For example,where the pressure p of the krypton gas is 50 Pa and the operatingfrequency of the ballast circuit 440 is 100 kHz, the correlation betweenthe input power and the total luminous flux is as shown in FIG. 2,whereby the discharge maintaining power P_(min) (W) can be obtained. Asthe power is lowered, the total luminous flux gradually decreases, andit becomes no longer possible to maintain a discharge at a particularpoint, with the total luminous flux becoming 0 eventually. P_(min) (W)is the input power at this particular point. Even a person skilled inthe art cannot know the point where a discharge can no longer bemaintained, except through actual measurement. P_(min) (W) is acritically significant point because the total luminous flux sharplydecreases past P_(min) (W).

As shown in FIG. 11, the present experiment proved that while a stabledischarge can be maintained even when the krypton gas pressure is set to40 to 50 Pa with commercially-available high-wattage-type (e.g., 100 W)electrodeless fluorescent lamps, it is difficult to maintain a dischargewith such a low gas pressure with electrodeless discharge lamps in whicha low-wattage (e.g., about 10 W) power is input to the discharge bulb.

The results shown in FIG. 11 will now be discussed in detail. Based onthe results shown in FIG. 11, the discharge maintaining power P_(min)(W) where the operating frequency is constant, e.g., 100 kHz, is about13.8 W for a krypton gas pressure of 120 Pa and about 11.6 W for akrypton gas pressure of 240 Pa. Thus, it can be seen that as thepressure p of the krypton gas decreases, the discharge maintaining powerP_(min) monotonically increases with the decrease in the pressure p.This tendency also applies when the operating frequency is 120 or 140kHz, where the discharge maintaining power P_(min) decreases as theoperating frequency f is increased.

Now, the experimental results will be discussed from the point of viewof designing an electrodeless fluorescent lamp. Consider a case where anelectrodeless discharge lamp having an emission power equivalent to thatof a self-ballasted electrodeless fluorescent lamp of 60 W is designedwith an operating frequency of 100 kHz and a krypton fill gas pressureof 120 Pa. Then, since the discharge maintaining power at 100 kHz and120 Pa is about 13.8 W based on the results shown in FIG. 11, it can beseen that it is impossible to design an electrodeless discharge lamp of10 W equivalent to an incandescent lamp of 60 W. Using the results ofFIG. 11, it can be seen that the operating frequency and the krypton gaspressure can be set to, for example, 140 kHz and 240 Pa, respectively,in order to produce an electrodeless fluorescent lamp equivalent to anincandescent lamp of 60 W.

As another example, the conditions and results of another experimentwill now be described.

A testing device for examining the operating characteristics of theelectrodeless discharge lamp used in the present experiment has the samebasic configuration as that used in the experiment described above,including the ballast circuit 440. Thus, the description of the commonparts will not be repeated for the sake of simplicity. The details ofthe electrodeless discharge lamp 260 used in the present experiment willnow be described.

The outer tube 101 of the discharge bulb 120 has a diameter D1 of 65 mmand a height H1 of 75 mm, and the inner tube 102 has an outer diameterD2 of 25.5 mm and a height H2 of 63 mm. Moreover, the core 103 of theinduction coil 130 has a length of 55 mm, an outer diameter D3 of 15.5mm and an inner diameter D4 of 8.5 mm, and the number of turns of thewinding 104 is 42. In this lamp, a heatsink is provided. Also in theexample described above, the lamp is provided with a heatsink.

In this experiment, five prototypes of the electrodeless discharge lamp260 were produced each having a krypton fill gas pressure p in the rangeof 200 Pa to 350 Pa, and the lamps were lit at an operating frequency fof 423 kHz (constant), so as to obtain the discharge maintaining powerP_(min) (W) of the electrodeless discharge lamp 260 for each gaspressure p. FIG. 12 shows an example of the results of this experiment.

Where the operating frequency was set to 423 kHz, the dischargemaintaining power P_(min) of the electrodeless discharge lamp 260 was9.3 W for a krypton gas pressure of 200 Pa and 7.9 W for a krypton gaspressure of 350 Pa, indicating that the discharge maintaining powerP_(min) was higher as the gas pressure p was lower. This is a similartendency to that seen in the results of the previous experiment.Moreover, it was found that as compared with the previous experiment,the discharge maintaining power decreases more significantly as theoperating frequency is increased.

Based on the results of the two experiments described above, thefollowing approximate expression was derived, which represents therelationship of the discharge maintaining power P_(min) (W) with respectto the krypton gas pressure p (Pa) and the operating frequency f (kHz).$\begin{matrix}{P_{\min} = {{A\quad\frac{1}{p^{2}}} + {B\quad\frac{1}{f^{2}}} + C}} & \left\lbrack {{Expression}\quad 5} \right\rbrack\end{matrix}$Note that the constants A, B and C were derived by the method of leastsquares to be A=4.0×10⁴, B=3.5×10⁴ and C=7.7.

FIG. 3 shows a three-dimensional plot of the data used for derivingExpression 5, where the x axis represents 1/p², the y axis represents1/f², and the z axis represents the discharge maintaining power P_(min).As a reference, FIG. 4(a) and FIG. 4(b) each show a two-dimensional plotbased on the data shown in FIG. 12.

It can be seen from FIG. 3 that the data points are nicely arrangedalong the plane of Expression 5 representing the discharge maintainingpower P_(min). Note that this plane is a critically significant planedistinguishing the “operatable” area and the “non-operatable” area fromeach other.

By using Expression 5, it is possible to obtain the minimum pressureP_(min) (Pa) of the krypton gas required for designing an electrodelessdischarge lamp operating device, where P (W) is the power input to thedischarge bulb 120 and f (kHz) is the operating frequency of the ballastcircuit. Specifically, the minimum pressure p_(min) (Pa) of the kryptonfill gas can be obtained by substituting P_(min) (W) and f in Expression5 with the value of the power input P (W) to the discharge bulb 120 andthe value of the operating frequency f (kHz), respectively, and thensolving the expression with respect to p.

Thus, based on Expression 2, where the power input to the discharge bulb120 of the electrodeless discharge lamp operating device is P (W) andthe device is operated at the operating frequency f (kHz), the pressurep (Pa) of the krypton gas filled in the discharge bulb should satisfythe following expression. $\begin{matrix}{{p \geq p_{\min}} = \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 6} \right\rbrack\end{matrix}$(where A=4.0×10⁴, B=3.5×10⁴ and C=7.7)

A measurement of the discharge maintaining power P_(min) with prototypesof electrodeless discharge lamps with a ballast circuit (invertercircuit) used in practice showed that the discharge maintaining powerP_(min) in actual electrodeless discharge lamps was lower by about 1.5 Woverall than the value obtained by the experiments described above.Therefore, when designing an actual electrodeless discharge lamp, it isconvenient to use the following expression, which is similar toExpression 6 but with a correction to C=6.2. $\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$(where A=4.0×10⁴, B=3.5×10⁴ and C=6.2)

FIG. 5 is a graphic representation of Expression 5. Specifically, it isa plot of the contour line of the discharge maintaining power P_(min),where the horizontal axis represents 1/p², an inverse square of thepressure, and the vertical axis represents 1/f², an inverse square ofthe frequency. Based on FIG. 5, once two of the wattage of theelectrodeless discharge lamp being designed, the rare gas pressure p andthe operating frequency f are set, the value of the remaining parametercan be obtained.

Note that when obtaining the value P_(min) of the minimum pressure ofthe krypton gas using Expression 1 in an actual design, it is needed tobe set to a value with some allowance taking into consideration thefluctuation of the power supply voltage, the characteristics degradationdue to aging in the electronic components used in the ballast circuit,etc.

An embodiment of the present invention, which is based on the researchresults described above, will now be described.

FIG. 6 schematically illustrates a configuration of an electrodelessdischarge lamp operating device according to the embodiment of thepresent invention. In order to facilitate the understanding of theconfiguration, FIG. 6 shows both the cross section of the discharge bulb120 and that of the core 103. Note that like elements to those alreadyillustrated in FIG. 1 will be give like reference numerals and will notbe further described below.

The electrodeless discharge lamp operating device of the presentembodiment includes the light-transmitting discharge bulb 120, aninduction coil (103, 104) for generating an electromagnetic field insidethe discharge bulb 120, and a ballast circuit 140 for supplying ahigh-frequency power to the induction coil. The operating frequency ofthe ballast circuit 140 is in the range of 80 kHz to 500 kHz. Where theoperating frequency of the ballast circuit 140 is f (kHz) and the powerinput to the discharge bulb 120 is P (W), the pressure p (Pa) of therare gas in the discharge bulb 120 satisfies the following relationship:$\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$(where A, B and C are constants having the following values: A=4.0×10⁴,B=3.5×10⁴ and C=6.2), and the power input P to the discharge bulb 120 is7 W at minimum and 22 W at maximum. The inside of the discharge bulb 120is filled with a rare gas including at least krypton and mercury, andthe induction coil including the core (103) and the winding 104 isinserted into a cavity portion provided in a portion of the dischargebulb 120.

The electrodeless discharge lamp operating device illustrated in FIG. 6is a so-called “self-ballasted electrodeless fluorescent lamp”. Theself-ballasted electrodeless fluorescent lamp includes a case 106supporting the discharge bulb 120 including the induction coil 130therein and made of an insulative plastic material for accommodating theballast circuit 140, and further includes a base 108 so that theelectrodeless discharge lamp operating device can be connected to aincandescent-lamp socket for receiving power supply. As illustrated inFIG. 6, the overall shape is an incandescent-lamp shape.

The discharge bulb 120 includes the outer tube 101 and the inner tube102. In the present embodiment, the discharge bulb 120 is filled withmercury and a krypton gas, and the inner surface of the discharge bulb120 is coated with a phosphor (not shown). Moreover, the exahust tube105 is connected to the inner tube 102.

The induction coil 130 is provided between the inner tube 102 of thedischarge bulb 120 and the exahust tube 105 for supplying anelectromagnetic energy for generating a discharge plasma inside thedischarge bulb 120. The induction coil 130 has a generally tubular shape(length: about 20 mm), and is formed by the winding 104 around the core103. The inductance of the induction coil 130 is about 120 (AH).Moreover, an Mn—Zn ferrite (relative magnetic permeability: about 2300)is used as the material of the core 103. An Mn—Zn ferrite is a ferritecontaining iron, manganese and zinc, and the induction coil core 103made of this ferrite is advantageous in that there is little magneticloss when the operating frequency of the ballast circuit is set to 80kHz to 500 kHz.

The ballast circuit 140 for supplying a high-frequency power to theinduction coil 130 includes electronic components forming the ballastcircuit, such as semiconductor devices (e.g., transistors), capacitors,resistors, inductors, etc., and a printed wiring board (not shown) onwhich these electronic components are arranged. The ballast circuit 140may have a circuit configuration as illustrated in FIG. 7, for example.

Specifically, the ballast circuit 140 may include a rectifier circuit220 electrically connected to a power supply (e.g., a commercial powersupply) 210, a smoothing capacitor 230, an inverter circuit 240 and aload resonant circuit 250. The inverter circuit 240 includes switchingdevices 241 and 242 and a driving circuit for driving the switchingdevices 241 and 242, and the load resonant circuit 250 includes aninductor 251 and capacitors 252 and 253.

The operation of the ballast circuit 140 will be briefly describedbelow. First, an alternating current from the commercial power supply210 is rectified at the rectifier circuit 220, and then smoothed at theelectrolytic capacitor (smoothing capacitor) 230. The output of theelectrolytic capacitor 230 is converted to a high-frequency current atthe inverter circuit 240, and a high-frequency power is supplied to thedischarge bulb 120 via the load resonant circuit 250.

With the self-ballasted electrodeless fluorescent lamp of the presentembodiment, a light output equivalent to that of an incandescent lamp of60 W can be obtained. When designing the self-ballasted electrodelessfluorescent lamp, the power input P to the discharge bulb 120 was set to10 W (the rated power including the power loss at the ballast circuitwas 11 W). The frequency of the high-frequency power supplied to thedischarge bulb 120, i.e., the operating frequency f of the ballastcircuit 140, was set to 400 kHz. Under such a condition, the requiredpressure p of the krypton fill gas was obtained.

Where the operating frequency f of the self-ballasted electrodelessfluorescent lamp is 400 kHz and the power input P to the discharge bulbis 10 W, the krypton gas pressure p (Pa) required for maintaining astable discharge may be a pressure p that satisfies Expression 1, asdescribed above.

Note however that with an actual electrodeless discharge lamp operatingdevice, the power input to the discharge bulb 120 may be lower than therated power input due to various factors, such as the fluctuation of thevoltage supplied from the commercial power supply 210, the coupling losscaused by an external metal lighting fixture being in close vicinity,and the decrease over time in the capacitance of the electrolyticcapacitor used as the smoothing capacitor 230 for smoothing a current inthe ballast circuit 140. Taking these factors into consideration, whenactually designing an electrodeless discharge lamp operating device, itis preferred that the rare gas pressure is determined so that a plasmadischarge in the discharge bulb can be maintained even when the powerinput to the discharge bulb becomes smaller (e.g., 70%) than the ratedpower input, in view of actual use of the device. Therefore, it is asafer design to obtain the pressure p by using a value that is 70% ofthe rated power input P to the discharge lamp as the value of thepressure p required for the krypton gas in Expression 3 above.

Using f=400 (kHz) and P=10×0.7 (W) in Expression 1, the minimum pressurep_(min) required for the krypton gas is about 250 (Pa). Therefore, inthe self-ballasted electrodeless fluorescent lamp of the presentembodiment, the pressure p of the krypton gas may be set to about 250(Pa) or more. Similarly, where the power input P to the discharge bulb120 is set to 18 W (where the rated power including the power loss ofthe ballast circuit is set to 20 W) when designing the device in orderto obtain a light output equivalent to that of an incandescent lamp of100 W, the pressure p of the krypton gas may be set to about 80 (Pa) ormore.

On the other hand, it is important in determining a krypton gas pressureto make the efficiency of the electrodeless discharge lamp operatingdevice as high as possible. In view of this, the present inventorsproduced prototypes of self-ballasted electrodeless discharge lamps inwhich the power input to the discharge bulb is 10 W to 20 W, andconducted experiments on the efficiency thereof.

The results indicated that for 20 W, the efficiency of theself-ballasted electrodeless fluorescent lamp was highest when thekrypton gas pressure was set to about 50 (Pa) and, for 10 W, it wasdifficult to maintain a discharge when the krypton gas pressure was 100Pa or less, with the efficiency decreasing as the pressure wasincreased. In either case, the highest efficiency point exists in anarea below the above-described rare gas pressure determined while takinginto consideration the power fluctuation. Therefore, it is preferredthat the rare gas is filled at the lowest possible pressure with which adischarge can be maintained.

This will now be discussed in greater detail based on the results of oneexperiment shown in FIG. 8. The experimental results shown in FIG. 8 arethose obtained under a condition where the lamp input was 10 W and theoperating frequency was 400 kHz. Since the lamp input is as low as 10 W,it is not possible to maintain a stable discharge if the gas pressure is150 Pa or less. Therefore, in FIG. 8, the portion in the area of 150 Paor less, denoted by a broken line, is obtained by extrapolation usingdata for a lamp input of 18 W.

As shown in FIG. 8, under a condition where 100% krypton is used, thelamp input is 10 W and the operating frequency is 400 kHz, theefficiency is highest at a gas pressure of about 50 Pa, and theefficiency decreases rapidly for pressure values below the gas pressureand decreases gradually for pressure values above the gas pressure. Thisis because in a lower-pressure area, electrons move more easily, therebyincreasing the loss (diffusion loss) in which electrons are taken by thetube wall, thus decreasing the efficiency and, in a higher-pressurearea, the loss due to elastic scattering, which does not contribute tothe light emission, increases, thus decreasing the efficiency.

While the efficiency is highest at a gas pressure of about 50 Pa asdescribed above, a stable discharge cannot be maintained at such a gaspressure. Therefore, in a gas pressure range where a stable dischargecan be maintained, the efficiency is higher as the pressure is lower. Asdescribed above, a gas pressure of 250 Pa or more is required when amargin is provided taking into consideration the fluctuation of thepower supply voltage, the decrease in the power due to degradation ofcircuit elements, and variations in the gas pressure during themanufacturing process. Taking both of these into consideration, anoptimal design value is 250 Pa under the condition of this experiment.

Taking the above into consideration, in the present embodiment, thepressure of the krypton fill gas is set to 250 (Pa) to be on the saferside with respect to the gas pressure. Note that the present inventorshave actually produced prototypes of the electrodeless discharge lampoperating device of the present embodiment, and confirmed that a stabledischarge can be maintained without flickering.

As described above, in the electrodeless discharge lamp device of thepresent embodiment, the pressure of krypton gas filled in the bulb wasset to about 250 (Pa). Note that Japanese Laid-Open Patent PublicationNo. 55-60260 discloses a condition of 0.1 to 5 mmHg (about 13 to about670 Pa) for the partial pressure of the krypton gas filled in anelectrodeless fluorescent lamp where the operating frequency of theballast circuit is set to about 10 MHz. However, the operating frequencyof the ballast circuit as disclosed in this publication is totallydifferent from that of the electrodeless discharge lamp device of thepresent embodiment, indicating that the technical concept of thepublication is basically significantly different from that of thepresent invention. Moreover, in Japanese Laid-Open Patent PublicationNo. 55-60260, the krypton gas pressure is determined from a point ofview of obtaining a level of startability similar to that obtained withan argon gas, and the publication fails to describe maintaining a stabledischarge. In addition, the startability of an electrodeless dischargelamp and the discharge stability thereof are different from each otherin terms of the discharge mechanism, and experimental results on thestartability does not dictate the discharge stability condition.

Note that with the configuration of the present embodiment, the powerinput P_(min) (W) to the discharge bulb required for maintaining adischarge generally decreases as the operating frequency f (kHz)increases. However, because changing the operating frequency f (kHz) toa frequency in the MHz range not only makes the driver for driving theinverter circuit more expensive, but also complicates theelectromagnetic interference (EMI) countermeasure, a range of 80 to 500(kHz) is preferably used.

Next, the operation of the self-ballasted electrodeless fluorescent lampof the present embodiment will be briefly described below. When acommercial alternating-current power is supplied to the ballast circuit140 via the base 108, the ballast circuit 140 converts the commercialalternating-current power to a high-frequency alternating-current powerand supplies the converted power to the winding 130. The frequency ofthe alternating current supplied by the ballast circuit 140 is, forexample, 80 to 500 kHz, as described above, and the supplied power is,for example, 7 to 22 W. Receiving the supply of a high-frequencyalternating-current power, the winding 130 forms a high-frequencyalternating magnet field in the space therearound. Then, an inducedelectric field is produced so as to be perpendicular to thehigh-frequency alternating magnet field, and the light-emitting gasinside the discharge bulb 120 is excited to emit light, therebyobtaining light emission in the ultraviolet range or the visible range.Light emission in the ultraviolet range is converted by a phosphor (notshown) formed on the inner wall of the discharge bulb 120 to lightemission in the visible range (visible light). Note that a lamp may beprovided without the phosphor so that light emission in the ultravioletrange (or light emission in the visible range) is used as it is. Lightemission in the ultraviolet range is produced primarily from mercury.More specifically, when a high-frequency current is passed through theinduction coil (103, 104) brought into the vicinity of the dischargebulb 120, an induced electric field formed byelectromagnetically-induced lines of magnetic force causes mercury atomsand electrons in the discharge bulb 120 collide with each other, therebyobtaining an ultraviolet radiation from the excited mercury atoms.

The frequency of the alternating current supplied from the ballastcircuit 140 will now be further described. In the present embodiment,the frequency of the alternating current supplied from the ballastcircuit 140 is in a relatively low frequency range of 1 MHz or less(e.g., 80 to 500 kHz), as compared with 13.56 MHz, being an ISM band, ora few MHz, which are commonly used in practice. The reason for using afrequency in the low frequency range is as follows. First, if the deviceis operated in a relatively high range such as 13.56 MHz or a few MHz,there is required a large-sized noise filter for suppressing the linenoise from the ballast circuit 140, thus increasing the volume of theballast circuit 140. Moreover, since very strict regulations are imposedby laws on high-frequency noise, if noise radiated or propagated fromthe lamp is high-frequency noise, it is necessary to provide anexpensive shield in order to observe the regulations, which presents asignificant hindrance to reducing the cost. If the device is operated ina frequency range of about 80 kHz to 500 MHz, inexpensive,commonly-available components used as electronic components in commonelectronic appliances can be used as members forming the ballast circuit140, and small-sized members can be used, whereby it is possible toreduce the cost and the size, thus providing a significant advantage.

Note that in a self-ballasted electrodeless fluorescent lamp or anelectrodeless discharge lamp operating device in which the operatingfrequency is set to 80 kHz to 500 kHz, if the krypton gas pressureexceeds 350 Pa, the starting voltage of the lamp increases so much thatit is difficult to start operating the lamp. Therefore, in view of thestartability, it is preferred that the upper limit of the krypton gas is350 Pa.

Where the low-wattage electrodeless discharge lamp operating device orthe low-wattage self-ballasted electrodeless fluorescent lamp of thepresent embodiment is operated by being connected to a commercial powersupply, it is possible to prevent a discharge from being unstable ordiscontinued even if the power supply voltage fluctuates or thecapacitance of the electrolytic capacitor decreases. As a result, astable discharge can be maintained.

The configuration of the present embodiment is not limited to theexample illustrated above, but may be modified. For example, while a100(%) krypton gas is used in the above example, a mixed gas includingargon or xenon in addition to krypton may be used. When xenon is mixedin, the power input to the discharge bulb required for maintaining adischarge is smaller than that with 100(%) krypton. Mixing in argon wasexperimented in greater detail as follows.

First, a research on the lamp efficiency will be described. As shown inFIG. 9, an examination was made as to how the lamp efficiency changeswhen the mixing ratio (partial pressure ratio) between a krypton gas andan argon gas is varied while fixing the total gas pressure to 200 Pa and250 Pa. The conditions include a lamp input of 11 W, and an operatingfrequency of 480 kHz.

Where the total gas pressure was 200 Pa, the maximum value of the totalluminous flux (an indicator of the lamp efficiency) is obtained when theargon gas is mixed in to a proportion of about 10%, and the totalluminous flux decreases rapidly if the argon gas mixing ratio exceeds20%. Therefore, in this case, the argon gas mixing ratio is preferably20% or less. Note that in the range of 0 to 20%, the total luminous fluxdoes not change substantially.

On the other hand, where the total gas pressure is 250 Pa, the maximumvalue of the total luminous flux is obtained when the argon gas is mixedin to a proportion of about 20%, and the total luminous flux decreasesrapidly if the argon gas mixing ratio is lower than 10% or higher than30%.

In order to obtain a high lamp efficiency when the total gas pressure is200 to 250 Pa, taking into consideration variations in the total gaspressure during the manufacturing process, etc., the argon mixing ratiois preferably 10 to 30% according to the results above.

Next, a research on the running-up of a lamp lighting will be described.For example, where the total gas pressure is 200 Pa, mixing in an argongas is advantageous in that the brightness running-up after the startingis improved although the lamp efficiency during the stable operatingperiod is not improved substantially, as shown in FIG. 9.

FIG. 10 shows how the proportion of the luminous flux one second afterthe starting to that during the stable operating period (an indicator ofthe running-up characteristics) changes when the mixing ratio (partialpressure ratio) between a krypton gas and an argon gas is varied under acondition where the lamp input is 11 W, the operating frequency is 480kHz and the total gas pressure is 200 Pa.

As shown in FIG. 10, in the argon gas mixing ratio range of 0% to 50%,the luminous flux one second after the starting increases as the argongas mixing ratio is increased. This is because an argon gas has a higherion voltage than a krypton gas, thereby increasing the lamp impedanceimmediately after the starting (where the discharge bulb is cool andthere is little mercury vapor), making it more likely that the power isinput at a higher level. Note that if the argon gas mixing ratio exceeds20%, the luminous flux one second after the starting does not increasesignificantly.

Based on the researches on the lamp efficiency and the running-upcharacteristics as described above, the argon gas mixing ratio ispreferably 10% or more and 50% or less. Moreover, if the argon gasmixing ratio is 50% or less, there is substantially no divergence fromExpression 5. If the mixing ratio exceeds 50%, the power input to thedischarge bulb required for maintaining a discharge becomes higher thanthat obtained with 100(%) krypton. Also for these reasons, it ispreferred that the argon gas mixing ratio is 10% or more and 50% orless.

With the self-ballasted electrodeless fluorescent lamp of the presentembodiment, the shape of the electrodeless discharge lamp 260 is anincandescent-lamp shape. However, the shape may of course be any othersuitable shape such as a spherical shape or a tubular shape. Moreover,while the self-ballasted electrodeless fluorescent lamp has an outertube diameter D1 of 65 mm and an inner tube diameter D2 of 25.5: mm inthe present embodiment, similar effects can be obtained also when thediameter D1 of the outer tube is set in a range of 55 to 95 mm and theouter diameter D2 of the inner tube is set in a range of 20 to 30 mm.Moreover, while the number of turns of the winding 104 is 66 in thepresent embodiment, the number of turns may be 30 to 70.

When the self-ballasted electrodeless fluorescent lamp is in the lampdischarge period, if the temperature of the core 103 of the inductioncoil 130 increases so that the temperature of the magnetic material usedas the core 103 exceeds a certain critical temperature (the Curietemperature), the magnetic permeability decreases, and the discharge maybe discontinued. A heat radiating structure for preventing such an eventmay be employed, e.g., a structure as disclosed in Japanese UtilityModel Publication for Opposition No. 6-6448, i.e., a structure includinga rod-shaped heat conducting material (made of copper) inserted into atubular core, and a plate connected to one end of the heat conductingmaterial, with the plate being brought into contact with the lamp case(jacket) so as to release heat to the outside. Moreover, a heatradiating structure for preventing shortening of the lifetime due to theincrease in the temperature of the electrolytic capacitor 230 used inthe ballast circuit may be employed, e.g., a structure as disclosed inJapanese Laid-Open Patent Publication No. 10-112292, i.e., a structureincluding a heat insulating structure between the discharge bulb and theelectrolytic capacitor so that heat from the discharge bulb side is nottransferred to the electrolytic capacitor.

In addition, while the exahust tube 105 is provided inside the core 103of the induction coil 130 in the electrodeless discharge lamp of thepresent embodiment, the exahust tube 105 may be attached to any othersuitable location. For example, it may be attached to a tip portion ofthe outer tube 101 and pinch-sealed. Moreover, while the inner surfaceof the discharge bulb 120 is coated with a phosphor in theself-ballasted electrodeless fluorescent lamp of the present embodiment,the phosphor is not limited to those for general lighting, but mayalternatively be a phosphor emitting an action spectrum for an erythemaleffect or a phosphor emitting a plant-growing action spectrum. Note thatno phosphor coating may be used as described above so as to utilize agermicidal effect by an ultraviolet radiation.

Furthermore, if the self-ballasted electrodeless fluorescent lamp of thepresent embodiment is coated with a monochromatic phosphor such as aY₂O₂:Eu phosphor (red), a CeMgAl₁₁O₁₉:Tb phosphor (green) or aBaMg₂Al₁₆O₂₇:Eu²⁺ phosphor (blue), it may be used as a replacement foran incandescent lamp of a display device.

While the present embodiment is directed to a self-ballastedelectrodeless fluorescent lamp including a discharge bulb, a ballastcircuit and a base integrated as one unit, the present invention cansimilarly be carried out with an electrodeless discharge lamp operatingdevice in which the ballast circuit is separately provided from thedischarge bulb.

According to the present invention, the operating frequency of theballast circuit is in the range of 80 kHz to 500 kHz, and where theoperating frequency of the ballast circuit is f (kHz), and the powerinput to the discharge bulb is P (W), the pressure p (Pa) of the raregas in the discharge bulb satisfies the relationship of the followingexpression: $\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$(where A, B and C are constants having the following values: A=4.0×10⁴,B=3.5×10⁴ and C=6.2), and the power input P to the discharge bulb is 7 Wat minimum and 22 W at maximum, whereby it is possible to prevent adischarge from being unstable or discontinued, thus maintaining a stabledischarge.

INDUSTRIAL APPLICABILITY

The electrodeless low-pressure discharge lamp operating device and theself-ballasted electrodeless fluorescent lamp of the present inventionhave a high industrial applicability in that they are useful asindustrial and household lighting and, particularly, they can be usedstably over a long period of time and can be used with a small powerconsumption when used as an incandescent lamp replacement.

1. An electrodeless low-pressure discharge lamp operating device,comprising: a light-transmitting discharge bulb filled with a rare gasincluding at least krypton and mercury; an induction coil including acore and a coil wound around the core for generating an electromagneticfield inside the discharge bulb; and a ballast circuit for supplying ahigh-frequency power to the induction coil, wherein: an operatingfrequency of the ballast circuit is in a range of 80 kHz to 500 kHz, andwhere the operating frequency of the ballast circuit is f (kHz) and apower input to the discharge bulb is P (W), a pressure p (Pa) of therare gas in the discharge bulb satisfies a relationship of a followingexpression: $\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$ (where A, B and C are constants having the followingvalues: A=4.0×10⁴, B=3.5×10⁴ and C=6.2); and the power input P to thedischarge bulb is 7 W at minimum and 22 W at maximum.
 2. Theelectrodeless low-pressure discharge lamp operating device of claim 1,wherein the core of the induction coil contains iron, manganese andzinc.
 3. The electrodeless low-pressure discharge lamp operating deviceof claim 1 or 2, wherein: the rare gas filled in the discharge bulbincludes argon; and the argon is 10% or more and 50% or less of the raregas.
 4. A self-ballasted electrodeless fluorescent lamp, comprising: alight-transmitting discharge bulb filled with a rare gas including atleast krypton and mercury; an induction coil including a core and a coilwound around the core and being inserted into a cavity portion providedin a portion of the discharge bulb; a ballast circuit for supplying ahigh-frequency power to the induction coil; and a base electricallyconnected to the ballast circuit, wherein: an operating frequency of theballast circuit is in a range of 80 kHz to 500 kHz, and where theoperating frequency of the ballast circuit is f (kHz) and a power inputto the discharge bulb is P (W), a pressure p (Pa) of the rare gas in thedischarge bulb satisfies a relationship of a following expression:$\begin{matrix}{p \geq \sqrt{\frac{A}{P - \frac{B}{f^{2}} - C}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$ (where A, B and C are constants having the followingvalues: A=4.0×10⁴, B=3.5×10⁴ and C=6.2); and the power input P to thedischarge bulb is 7 W at minimum and 22 W at maximum.
 5. Theself-ballasted electrodeless fluorescent lamp of claim 4, wherein thecore of the induction coil contains iron, manganese and zinc.
 6. Theself-ballasted electrodeless fluorescent lamp of claim 4 or 5, wherein:the rare gas filled in the discharge bulb includes argon; and the argonis 10% or more and 50% or less of the rare gas.
 7. The electrodelesslow-pressure discharge lamp operating device of claim 1, wherein amaximum value of the power input P is 13 W or less.
 8. Theself-ballasted electrodeless fluorescent lamp of claim 4, wherein amaximum value of the power input P is 13 W or less.